Known devices for controlling the gripper straps on continuous weft feed looms consist, as e.g. shown by FR-A-2303876, of a connecting rod-crank drive mechanism for translating along a straight guide a slide fitted with means for engaging a variable-pitch worm screw mounted in idle manner parallel to, and on a support integral with, the guide. The screw is thus rotated, by the reciprocating translatory movement of the slide, so as to transmit a reciprocating rotatory movement, to a toothed pinion fitted angularly integral with the screw, and which in turn, and in known manner, controls translation of a flexible gripper strap wound partially about the pinion.
Known devices of the aforementioned type, though they do provide for varying the law of motion of the gripper strap by means of a relatively straightforward mechanical operation on the device itself, and for gradually taking up any slack between the screw profiles (or threads) and means for reading/engaging the same, are nevertheless mechanically complex, expensive to produce, and unreliable, particularly as regards the reading/engaging means which normally consist of idle rollers with an oblique axis in relation to the screw.